Flowmeters may be used in the induction tract and/or exhaust tract of combustion engines for measuring volumetric flow, for measuring mass flow or for measuring velocity. Such ultrasonic transducers are provided especially for flow measurement in air; however, in principle, other fluid media, thus gases and/or liquids may also be used. In the automotive sector, air-quantity signals and/or air-mass signals may be derived from such an ultrasonic flow measurement within a system control of a combustion engine.
Ultrasonic transducers may be used which are able both to emit ultrasonic waves into a fluid medium and to receive ultrasonic waves from the fluid medium. In this context, ultrasonic signals are usually transmitted through the flowing fluid medium from an emitter to a receiver, and in so doing, propagation time, propagation-time differences, phases, phase differences or perhaps combinations of these and/or other measured quantities are recorded. These measured quantities or signals are influenced by the flow of the fluid medium. It is possible to infer the flow velocity of the fluid medium from the degree of the influencing of the propagation time. Various measuring systems and evaluation principles are possible, e.g., ultrasonic flowmeters having one, two or more ultrasonic transducers. Examples of ultrasonic transducers are discussed in DE 10 2007 010 500 A1, in DE 42 30 773 C1 and in EP 0 766 071 A1. The ultrasonic transducers discussed in this related art may also be modified according to the scope of the exemplary embodiments and/or exemplary methods of the present invention, so that, for example, reference may be made to these printed publications for possible embodiments.
However, one problem of many familiar ultrasonic flowmeters, at least when working with gaseous media, lies in comparatively low ultrasonic signal amplitudes. In particular, this is because the vibrational energy generated by customary ultrasonic generators, e.g., a piezoceramic, must overcome a high acoustic impedance difference, as a rule, approximately a factor of 6×105, during the coupling into the medium to be measured. Due to this, as a rule, approximately 99.9995% of the sound energy on the way from a piezoceramic into air is reflected back at the corresponding boundary surface, and is not usable for the measurement. The same reflection loss occurs again at the second receiving transducer element, which may also be identical to the first transducer element. In order to improve the acoustic coupling between the transducer element and the fluid medium to be measured, usually matching members are therefore used, e.g., in the form of one or more matching layers, which promote vibration coupling between the piezoelectric transducer element and the surrounding fluid medium. For instance, ultrasonic transducers are familiar which have sound-radiating resonance members or matching members, such as a metallic membrane or a /4-impedance-matching layer.
In M. I. Haller et al.: 1-3 Composites for Ultrasonic Air Transducers, IEEE 1992 Ultrasonics Symposium, 937 to 939, a matching member made of micromechanically produced Kapton® (a polyimide material by DuPont) is discussed. In that case, a column array of polyimide is produced with the aid of an oxygen plasma. However, the micromechanical method described there is technically extremely complex, and as a rule, is therefore not suitable for high-volume applications.
Ultrasonic transducers, particularly in the application areas indicated, must normally satisfy a multitude of boundary conditions. One important requirement is, in particular, a resistance of the ultrasonic transducers to media, especially with respect to the fluid media in which the ultrasonic transducers are intended to be used. Thus, for example, ultrasonic transducers should represent a robust ultrasonic air-mass measurement, e.g., as replacement for or addition to conventional thermal air-mass measurements, and should represent a key element for achieving tough exhaust-emission standards such as the EU6 exhaust-emission standard. However, for this purpose, the ultrasonic transducers must be usable in an induction atmosphere of a motor vehicle, for instance, in which they are exposed to environmental influences including moisture, oil, dust, fuels, exhaust components and/or further chemicals.
In addition, many ultrasonic transducers are used in areas in which the fluid medium is under high pressure. For example, they may be used in the induction tract downstream of turbochargers, and compressive loads of, for instance, 2 to 6 bar may occur. In order to ensure such media resistance and/or pressure resistance, the related art discusses ultrasonic transducers in which the sound-radiating surface or sound-receiving surface is an integral component of a transducer housing and/or of a flow pipe. The printed publications EP 0 766 071 A1 and DE 42 30 773 C1 cited above are examples thereof.
A further demand on customary ultrasonic transducers is thermal stability. Ultrasonic transducers can be used in very large temperature ranges. The encapsulation by a suitable housing described above offers a solution, at least to a great extent, with respect to this requirement, as well. However, in many cases, the encapsulation in a housing described in the related art gets into a conflict of aims with respect to a third requirement which must be fulfilled while at the same time maintaining the media/pressure resistance and the thermal stability, namely, the requirement with respect to suitable acoustic properties. On their part, these acoustic properties are subdivided into two requirements, namely, the requirement that there must be good coupling of the ultrasonic waves between the piezoelectric transducer element and the fluid medium, for which purpose, for example, one or more of the matching layers described above are used.
At the same time, however, there must be good decoupling with respect to the propagation of structure-borne noise in order, for example, to protect the piezoelectric transducer element from such structure-borne-noise propagation, e.g., via a flow pipe or sensor housing. This propagation of structure-borne noise may stem from external interference sources, or else be caused by the ultrasonic transducer currently transmitting, and may overlap in the currently receiving ultrasonic transducer with the sound transmitted through the fluid medium, and thus lead to measuring errors.
However, if the sound-radiating or sound-receiving surface of the ultrasonic transducers is an integral part of the transducer housing and/or of the flow pipe, then usually there is no such decoupling. Therefore, to decouple structure-borne noise, the related art frequently uses molded parts or potting areas made of elastomer materials, silicone materials, polyurethane materials, flexibilized epoxy materials or foamed materials. These decoupling materials are usually incorporated between the ultrasonic transducer and the flow pipe or sensor housing, and on their part, are exposed directly to the media. On their own, elastomers more resistant to media or moisture, like, for example, fluorinated materials, are in turn relatively hard, and therefore are only suitable for the decoupling when the transitions between the decoupling material and the transducer or flow pipe or sensor housing have a relatively small cross-sectional area like, for example, in the case of an O-ring which, in first approximation, permits a linear and therefore small transition region. However, such restrictions with regard to the geometry of the decoupling element lead to an unsatisfactory compromise with respect to decoupling efficacy and pressure resistance.